Cell entry mechanisms of coronaviruses : To detect the conformational changes of viral fusion protein.

以往的研究表明，在S介导的细胞-细胞或病毒-细胞膜融合过程中，需要弹性蛋白酶、组织蛋白酶或TMPRSS2对S蛋白进行蛋白水解性切割。对于流感病毒，也有报道用酶激活病毒糖蛋白(GP)。流感病毒和SARS-CoV最明显的区别是病毒复制阶段，病毒糖蛋白被蛋白酶切割。在流感病毒中，蛋白酶在成熟过程中会在GP上进行简单的切割。相反，SARS-CoV S蛋白在受体诱导的构象变化后被蛋白酶切割。S蛋白中的蛋白酶裂解部位被认为是在受体结合后才暴露出来的。为了支持这一模型，我们报道了小鼠2型肝炎病毒的S蛋白与SARS冠状病毒的S蛋白高度相似，需要通过顺序受体结合和蛋白降解介导的两步构象变化来激活膜融合。这种机制允许对融合进行严格的时间控制，因为它保护激活的切割位点不被过早的蛋白分解，同时允许在与靶细胞上的受体结合时进行有效的切割。

Previous studies have demonstrated that the SARS-CoV S protein requires proteolytic cleavage by elastase, cathepsin or TMPRSS2 for S-mediated cell-cell or virus-cell membrane fusion. Activation of viral glycoprotein (GP) by protease also has been reported for influenza virus. The most distinctive difference between influenza virus and SARS-CoV is the stage during virus replication in which viral glycoproteins are cleaved by proteases. In influenza virus, the protease makes a simple cut in the GP during maturation. In contrast, SARS-CoV S protein is cleaved by the protease following receptor-induced conformational changes. The protease cleavage site in S protein is thought to be exposed only after receptor binding. In support of this model, we reported that the S protein of mouse hepatitis virus type 2 (MHV-2), which is highly similar to the S protein of SARS-CoV, requires two-step conformational changes mediated by sequential receptor binding and proteolysis to be activated for membrane fusion. Such a mechanism allows for tight temporal control over fusion by protecting the activating cleavage site from premature proteolysis yet allowing efficient cleavage upon binding to the receptor on target cells.